Comparison of Two Sampling Methods for Estimating the Abundance and Distribution of Mysis relicta

Vertical net hauls taken at night yielded a population estimate for Mysis relicta of 5 300 000 ± 1 060 000 (P ≤ 0.05) while quadrat counts done by SCUBA divers in the daytime fixed the population size at 1 100 000 ± 170 000. Observations by divers suggested that the behavior and distribution of Mysis during the day resulted in an underestimate of their abundance by the quadrat method. Net hauls showed that at night mysids migrated both vertically to the base of the epilimnion and horizontally within the metalimnion. A vertical tow rate of [Formula: see text] m∙s −1 used for the nighttime population estimates lay within the optimum sampling speed of the net. Repeated sampling at a single station had little or no effect on catch totals for up to nine consecutive hauls. Beyond this number some local depletion was noted. A modified vertical tow net which sampled on descent then closed once contact was made with the substrate performed no better than a standard net except when the shallowest stations were sampled repeatedly. Temporal variation in the catch of young-of-the-year Mysis at shallow stations, however, contributed at most ± 8% error to the whole lake population estimate.